1. Field of the Invention
The present invention relates to an illumination device and a liquid crystal display device, and more particularly, to the configuration of an illumination device that achieves a highly uniform distribution of the amount of emitted light with a single light source, and to the configuration of a liquid crystal display device using the illumination device.
2. Description of the Related Art
Front lights of reflective liquid crystal display devices have been formed in a unit including a light source, an intermediate light guide, a light guide plate, and a case member having a reflective inner surface to hold these members together.
FIG. 13A is a perspective structural view of a conventional liquid crystal display device, and FIG. 13B is a plan view of a front light provided in the liquid crystal display device shown in FIG. 13A, as viewed from the viewing side. The liquid crystal display device shown in these figures includes a liquid crystal display unit 120, and a front light 110 disposed on the front side of the liquid crystal display unit 120. Although not shown in detail, the liquid crystal display unit 120 is a reflective liquid crystal display unit that performs display by reflecting light incident from its front side, and holds a liquid crystal layer 123 between an upper substrate 121 and a lower substrate 122 opposing each other. By controlling the alignment state in the liquid crystal layer 123, the light transmitting state is changed to perform display.
The front light 110 includes a flat light guide plate 112, a bar-shaped intermediate light guide 113 disposed at a side face (light incident face) 112a of the light guide plate 112, and a light emitting element 115 formed of a point light source, such as a white LED (Light Emitting Diode), and disposed at one end face (left end face in the figures) 113g of the intermediate light guide 113. An upper surface of the light guide plate 112 serves as a reflecting surface 112c on which a plurality of prism grooves 114 of wedge-shaped cross section are formed in parallel and in stripes in plan view, and a lower surface thereof serves as an emergent surface 112b from which illumination light for illuminating the liquid crystal display unit 120 is emitted. Each of the prism grooves 114 is composed of a gently inclined face 114a and a sharply inclined face 114b. The inclination angle θ1 of the gently inclined face 114a is set at a fixed value within the range of 5° to 35°, and the inclination angle θ2 of the sharply inclined face 114b is set at a fixed value larger than the inclination angle θ1 of the gently inclined faces 114a. The pitch P of the prism grooves 114 is fixed in the plane of the reflecting surface 112c. The depth of the prism grooves 114 is also fixed in the plane of the reflecting surface 112c. The length of an outer side face 113a of the intermediate light guide 113 is equal to the length of the light incident face 112a of the light guide plate 112, and the end face 113g of the intermediate light guide 113 is flush with an end face 112g of the light guide plate 112 close to the light emitting element 115. A prism face 113f is formed on the outer side face 113a of the intermediate light guide 113 (remote from the light guide plate 112) in the lengthwise direction of the outer side face 113a (from the end face 113g close to the light emitting element 15 toward an end face 113h on the remote side) to reflect light propagating inside the intermediate light guide 113 in order to change the propagating direction.
Therefore, in the front light 110 shown in FIG. 13, light emitted from the light emitting element 115 is introduced into the intermediate light guide 113 through the end face 113g, is caused by the prism face 113f to change the propagating direction, and is introduced into the light guide plate 112 through the side face 112a. The light is reflected by the upper reflecting surface (inner surface) of the light guide plate 112 having the prism grooves 114 to change the propagating direction thereof, and is emitted from the emergent surface (lower surface) of the light guide plate 112 toward the liquid crystal display unit 120.
In portable electronic devices such as personal digital assistants and portable game machines, since the battery driving time has a great influence on the ease of use, liquid crystal display devices used as display sections in the electronic devices have adopted a single-LED front light having only a single light emitting element, as in the front light 110 shown in FIGS. 13A and 13B, in order to reduce the power consumption of the front light. That is, the number of light emitting elements is limited to reduce the power consumption. Furthermore, with size reduction of the portable electronic devices, there has been a request to reduce the thickness of the front light to approximately 1 mm.
It is, however, substantially impossible for such a single-LED front light to uniformly and brightly illuminate a large display region having a diagonal size of several inches or more by a combination of the thin light guide plate and the single light emitting element. That is, in a case in which the light emitting element 115 is provided on one side of the front light 110 shown in FIG. 15, it is first necessary to make incident light uniform in the lengthwise direction of the side face 112a of the light guide plate 112 by the intermediate light guide 113 in order to uniformly guide light from the light emitting element 115 to the light guide plate 112. It is, however, difficult for the intermediate light guide 113 to make light incident on the light guide plate 112 uniform. Therefore, it is difficult to obtain uniform light emitted from the entire emergent surface 112b, and to uniformly illuminate the display region of the liquid crystal display unit 120 without causing variations in luminance. This sometimes reduces display visibility. In a particularly marked case, a dark portion 128 shaped like a strip in plan view is formed near the end face 112g of the light guide plate 112 on the side of the light emitting element 115, as shown in FIG. 13B. Consequently, the emitted light is not uniform, and visibility of the liquid crystal display device is reduced.
In a double-LED front light in which a light emitting element is also disposed at the other end face 113h (on the right side) of the intermediate light guide 113, the amount of light near the left end face 112g of the light guide plate 112 is supplemented by light emitted from the light emitting element, and brightness is increased. However, the power consumption is higher than in the single-LED front light.
While there is an increasing demand for a front light using a single light emitting element as a light source in this way, a thin front light that can uniformly and brightly illuminate a large area has not yet been achieved.